There were a number of changes between the two. I don’t have a full list, but you should see the differences if you put the two side by side. One thing I know was changed was the documentation for the Connection State API. Everything I posted in this article:

In Outlook2003 in the Scheduling page of a Calendar form, there is an option to display meeting details. Also when you position your mouse over a meeting, there is a tooltip displayed with data. Looking at the Free/Busy interfaces, I couldn’t find any way to return to Outlook other information than start/end time and status.

Is there a way to make Outlook display the details for the users invited to a meeting?